The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, a typical communication system 10 comprises an information source 12, a transmitter 13, a communication channel 20, a receiver 27, and a destination 28. The transmitter 13 comprises a source encoder 14, a channel encoder 16, and a modulator 18. The receiver 27 comprises a demodulator 22, a channel decoder 24, and a source decoder 26.
The information source 12 may be an analog source such as a sensor that outputs information as continuous waveforms or a digital source such as a computer that outputs information in a digital form. The source encoder 14 converts the output of the information source 12 into a sequence of binary digits (bits) called an information sequence u. The channel encoder 16 converts the information sequence u into a discrete encoded sequence v called a codeword. The modulator 18 transforms the codeword into a waveform of duration T seconds that is suitable for transmission.
The waveform output by the modulator 18 is transmitted via the communication channel 20. Typical examples of the communication channel 20 are telephone lines, wireless communication channels, optical fiber cables, etc. Noise, such as electromagnetic interference, inter-channel crosstalk, etc., may corrupt the waveform.
The demodulator 22 receives the waveform. The demodulator 22 processes each waveform and generates a received sequence r that is either a discrete (quantized) or a continuous output. The channel decoder 24 converts the received sequence r into a binary sequence u′ called an estimated information sequence. The source decoder 26 converts u′ into an estimate of the output of the information source 12 and delivers the estimate to the destination 28. The estimate may be a faithful reproduction of the output of the information source 12 when u′ resembles u despite decoding errors that may be caused by the noise.
In wireless communication systems, channel quality depends on the amount of noise and/or interference present in a channel. Channel quality is good when the amount of noise and/or interference present in the channel is low. Channel quality is bad when the amount of noise and/or interference present in the channel is high.
When channel quality is good, data may be reliably transmitted using codes having high data rates, where the number of redundant or parity bits used is low relative to the number of data bits. Conversely, when channel quality is bad, data may be reliably transmitted using codes having low data rates, where the number of redundant or parity bits used is high relative to the number of data bits.
Depending on channel quality, transmitters may use different modulation and coding schemes (MCSs) to transmit data. Each MCS may include a different code for encoding data and a different modulation scheme for modulating encoded data. Based on the code used, each MCS may have a different spectral efficiency, which is a ratio of data rate (also called coding rate) to channel bandwidth. Spectral efficiency is high or low when codes used have high or low data rates, respectively. Transmitters may adaptively select MCSs having high or low spectral efficiencies when channel quality is good or bad, respectively.